The Finance Sector Union (FSU) said it has retained senior counsel with the intention of lodging proceedings against NAB in the Federal court.
This comes after NAB launched an internal payroll review in December 2019 after concerns staff had not been paid correctly.
The bank has repaid $55 million to former and current employees to date, however this has only been for part time staff.
But, the FSU is alleging the bank still owes much more to full-time staff members.
FSU state secretary for Queensland Wendy Streets said the number of full-time staff potentially affected by the scandal is much greater.
“We had a member meeting six weeks ago and a resolution was passed to brief a barrister,” she said.
“There is case law which supports both sides but we believe we are right.”
The issue comes down to how each side is calculating “full time” work.
Full time workers at the bank at an executive level were paid over $100,000 a year. NAB’s position is that the extra hours in dispute above a 40-hour week amount to “reasonable additional work” given the seniority of the affected people.
NAB argues that it calculated executive pay correctly based on salary packages and the underpayment issue does not affect them.
“NAB determines the salary of our full time colleagues based on modelling against the Banking, Finance and Insurance Award (where relevant) and market analysis for equivalent full time roles,” the bank said in a statement.
“Any periods during which a colleague has worked in a full time capacity are unaffected by this issue as hours of work are not used to formulate their salary.”
NAB has so far identified 3,200 current and former colleagues impacted by the issue.
“We have paid all current colleagues and are now in the process of contacting former colleagues,” NAB said.
“The issues identified in the Payroll Review are not acceptable and we apologise to all current and former colleagues impacted. We are moving as quickly as possible to fix these issues and we want to make sure we get this right.”